Most cellular phones and other telecommunications and key-operated instruments use backlighting of their keypads to enable the user to view the identity and position of the keys in low or no ambient light conditions.
Typically, light-emitting diodes (LEDs) are mounted on a printed circuit board (PCB) and transmit light in an indirect manner to the keys of the keypad, such as via an underlying light guide or via individual fiber optics as disclosed in U.S. Pat. No. 5,097,396 issued Mar. 17, 1992 to J. Michael Myers. Alternatively, electro-luminescent panels are used to provide backlighting. However, the present methods are inefficient when considering the total power used with respect to the light output through the individual keys. Light may be lost through intermediate transmission, such as the light guide or optical fibers, through filters or attenuating layers, or overly broad areas of illumination. In addition, the present methods consume significant space.
With the push to smaller and smaller key-operated instruments and the need for longer and longer battery life, there remains a need for thin profile backlit keypads which use generated illumination efficiently. Such a backlit keypad assembly should direct virtually all of the light emitted by an LED to the corresponding key without light guides, fiber optic elements, or intermediate layers between the LED and the corresponding key.